Growth hormone (GH) is essential for postnatal growth in animals. At the cellular level GH elicits its actions by initially binding to a cell surface protein termed the growth hormone receptor (GHR). A variety of factors influence the expression of the GHR; GHR is virtually undetectable in fetal tissues while its expression increases dramatically during postnatal life. The factors controlling the developmentally-regulated expression of the GHR are not known. I have identified a novel enhancer element in the promoter-regulatory region of the murine GH receptor gene which exhibits developmentally-regulated DNA-binding activity in liver. Furthermore, the functional activity of this enhancer element in transient transfection assays is also developmentally-regulated. I HYPOTHESIZE that the identified enhancer element with its cognate trans-acting factor(s) play a role in the developmentally-regulated expression of the GHR gene. This proposal seeks SPECIFICALLY to elucidate the molecular mechanisms controlling the developmentally-regulated expression of the GHR gene. The experimental strategy detailed in this proposal is designed to test the role of the identified enhancer element in the developmentally-regulated expression of the GHR gene by site-directed mutagenesis, transient transfection assays of fetal and adult hepatocytes using chimeric reporter gene constructs, and by mapping of the DNase I hypersensitivity sites in the 5' flanking region. Following a preliminary biochemical characterization of the DNA-binding protein(s), the identity of these trans-acting factor(s) will be ascertained by cloning the cDNA encoding the genes for these putative transcription factor(s). These studies will (a) delineate the molecular basis for alterations in expression of the GHR gene in disorders of growth such as genetic short stature, intra-uterine growth retardation and malnutrition, enabling (b)the design of novel diagnostic tools and more effective treatment strategies for these disorders of growth and development in the human.